Currently, the typical method for monitoring the presence and concentration of airborne particles is a process wherein the air is continuously sampled and particles are detected by means of light scattering. This method is incorporated in most aerosol particle monitors which monitor the environment by Continuously illuminating air samples. The pattern of light scattered by any particle present in the air sample makes it possible to identify and count each particle on a real time basis. Airborne particle concentration (C) is then determined by dividing the number of particles counted (p) by the elapsed sampling time (t) multiplied by the detection flow rate (Q.sub.d) of the air sample through the monitor. Thus, by equation: ##EQU1##
Prior art particle monitors then display the particle count and related particle concentration in real time on a display means, usually a light emitting diode ("LED") or a liquid crystal display ("LCD") screen. Additionally, these devices allow the particle count and concentration to be continually outputted to a strip chart recorder, data logger and/or telemetry system.
Prior art real time particle monitors are able to
determine an environment's actual airborne particle concentration with statistical meaning when the concentration is relatively high, that is, when the total particle count (p) is sufficiently large for a particular sampling period (t) or if the sampling period is made sufficiently long. Problems arise when these prior art devices attempt to determine an environment's actual particle concentration when the concentration is very low, that is, when the total particle count (p) is very low for a particular sampling period (t). Specifically, a particle count of zero during a test sampling period is not necessarily equivalent to zero particle concentration. Similarly, a very low particle count, for example a count of 1, 2, or 3, will result in a particle concentration measurement with an implied large degree of uncertainty. As a result, when these devices are used in environments having a low airborne particle concentration, such as in clean rooms or when monitoring fibers in ambient outside air, they must be operated for long periods of time to achieve a statistically meaningful determination of particle concentration. For example, if an environment has an expected airborne particle concentration of 0.1 p/cc (particles per cubic centimeter), a particle monitor sampling air with a detection flow rate of 10 cc/min must be operated for approximately 100 minutes to determine the environment's actual particle concentration with adequate precision. When a particle monitor is used in an environment having an even lower particle concentration, the required operating time increases correspondingly,
Accordingly, it would be desirable to be able to determine over shorter periods of time an "upper limit" particle concentration with a significant degree of confidence. That is, to be able to state that the probable airborne particle concentration is below a certain value. Additionally, it would be desirable to be able to determine the required sampling time at a zero or low particle count to state with a significant degree of confidence that the actual particle concentration is below a certain value.
Prior art particle monitors are also unable to determine and printout particle concentrations having a constant, preferably user selected, fixed measurement precision. The inability to determine and printout particle concentrations having a selected measurement precision (i.e. to monitor an environment in a fixed precision mode) complicates particle monitoring when an environment's expected particle concentration is not known. In this situation, a required sampling time period (t) for a selected precision must first be determined. Then, the monitor must be operated for the predetermined time period to achieve an actual airborne particle concentration within the selected precision level. For example, to use a particle monitor in an environment where the expected particle concentration is not known, the user must first operate the monitor continuously to determine the time required to count 20 particles (t.sub.20). Then, to determine the required total sampling time (t) to achieve a particle concentration within a selected constant precision (PR) (at a confidence level of 95%), the user must solve the following equation: ##EQU2##
Finally, the user must operate the monitor for t minutes to determine the actual airborne particle concentration within the selected precision P (at a confidence level of 95%). Therefore, if it took 40 minutes to count 20 particles (t.sub.20 =40), and a user selected a measurement precision of .+-.20% (PR=20), the required total sampling time (t) is 200 minutes (200=40 (44.72/20).sup.2). The user must then operate the monitor for 200 minutes to determine the actual airborne particle concentration with a precision of20% at a confidence level of 95%.
The inability to output airborne particle concentrations in a fixed precision mode also prevents the user from obtaining more frequent concentration outputs when the airborne particle concentration increases, a situation where such information is most needed, or from obtaining concentration outputs having only a selected precision.